Nonaqueous electrolyte secondary batteries, such as lithium secondary batteries, are currently in wide use as batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal.
A nonaqueous electrolyte secondary battery, which is typified by a lithium secondary battery, has a high energy density, and may thus let a large electric current flow and generate heat in a case where a breakage in the battery or in the device using that battery has caused an internal or external short circuit. This risk has created a demand that a nonaqueous electrolyte secondary battery should prevent more than a certain level of heat generation to ensure a high level of safety.
Meanwhile, porous films are widely used as a separator provided between a cathode and an anode in a nonaqueous electrolyte secondary battery. Such a separator formed by a porous film has a function of preventing a short circuit between the cathode and the anode, and thereby significantly contributes to reliability (safety) of the nonaqueous electrolyte secondary battery. Patent Literatures 1 through 3, for example, each disclose a technique for preventing laminated electrodes from being shifted with respect to each other, so that a high level of safety can be achieved. According to the technique, a separator is shaped into a bag-like form by heat-sealing respective surfaces of two porous films by which a separator is formed.